In power transmission networks alternating current (AC) power is typically converted to direct current (DC) power for transmission via overhead lines and/or under-sea cables. This conversion removes the need to compensate for the AC capacitive load effects imposed by the transmission line or cable, and thereby reduces the cost per kilometer of the lines and/or cables. Conversion from AC to DC thus becomes cost-effective when power needs to be transmitted over a long distance.
The conversion of AC power to DC power is also utilized in power transmission networks where it is necessary to interconnect the AC networks operating at different frequencies.
In any such power transmission network, converters are required at each interface between AC and DC power to effect the required conversion, and one such form of converter is a voltage source converter (VSC).
One form of known voltage source converter is shown in FIG. 1a and includes six sets of series connected insulated gate bipolar transistors (IGBTs) 10 and anti-parallel diodes 12. The IGBTs 10 are connected and switched together in series to enable high power ratings of 10's to 100's of MW to be realized.
This approach however requires a complex and active IGBT drive, and requires large passive snubber components to ensure that the high voltage across the series strings of IGBTs 10 shares properly during converter switching. In addition the IGBTs 10 need to switch on and off several times at high voltage over each cycle of the AC supply frequency to control the harmonic currents being fed to the AC network 16. These factors lead to high losses, high levels of electromagnetic interference and a complex design.
Another known voltage source converter is shown in FIG. 1b and includes a multilevel converter arrangement. The multilevel converter arrangement includes converter bridges or cells 18 connected in series, each converter cell 18 including a pair of series connected IGBTs 20 connected in parallel with a capacitor 22. Each converter cell 18 is switched at a different time and such an arrangement eliminates the problems associated with the direct switching of series connected IGBTs because the individual converter cells 18 do not switch simultaneously and converter steps are comparatively small.
The capacitor 22 of each converter cell 18 must however have a high capacitive value to constrain the voltage variation at the capacitor terminals in the multilevel converter arrangement. Six DC side reactors 24 are also required to enable the parallel connection and operation of the converter limbs, and are primarily used to limit transient current flow between converter limbs.
These factors lead to expensive, large and heavy equipment with significant amounts of stored energy, making pre-assembly, testing and transportation of the equipment difficult.
In addition, the use of a large number of individually controlled cells means that a high number of fibre-optic communication channels are often required between ground level control and the high voltage converter. This is complex, expensive and requires sophisticated designs and very fast and accurate processing.